Acne occurs when our skin produces excess oil (sebum) in response to hormonal stimuli, most common during our teenage years. There's a widespread misbelief that certain foods can trigger acne outbreaks, though dermatologists disagree as there is simply no evidence that dietary factors have anything to do with it.
Having said this, the relationship between food and acne has not been well studied, and has not been researched at all since the 1970's. During this time, scientists tested four of the foods (and beverages) most commonly blamed for promoting pimples, which included peanuts, chocolate, milk and cola. As it turned out, neither one had any significant effect.
More recent evidence suggests that higher circulating insulin levels may promote stimuli for pimple production, which would suggest that low GI eating may help, though this is purely hypothetical. Whilst many people swear that certain foods cause acne breakouts or help prevent them, they do so based on subjective comparisons, which is why anecdotal evidence is notoriously unreliable.
Eating healthy is certainly good for you – just don't expect it to be a cure for acne. To clear up your concerns (and possibly your face) talk to your GP or dermatologist.
Magin P, Pond D, Smith W, Watson A. A systematic review of the evidence for 'myths and misconceptions' in acne management: diet, face-washing and sunlight. Fam Pract. 2005 Feb;22(1):62-70. Epub 2005 Jan 11.